1. Field of the Invention
This invention relates generally to a reconnaissance system and, more particularly, to a partial intercept LPI reconnaissance system to separately identify, track and provide DF (direction finding) on LPI RF (radio frequency) transmitters.
2. Discussion of the Related Art
Various reconnaissance systems are in use today for identifying, tracking and recording constant frequency RF signals from constant frequency RF transmitters. Some of these reconnaissance systems further provide DF capabilities for determining the location of the RF transmitters based upon the AOA (angle of arrival) of the RF signals. The AOA is typically calculated by utilizing relative phase measurements at a constant RF frequency gathered from multiple antenna pairs, known as baselines, in an antenna array. Once a set of baseline measurements from the antenna pairs have been gathered, the AOA calculation is made by utilizing a calibration table to determine RF transmitter locations.
In addition to tracking, recording, identifying and providing DF on constant frequency RF transmitters, there is a growing need to identify, track and provide DF on LPI RF transmitters. These LPI RF transmitters utilize frequency agile techniques which makes them difficult to intercept and, thus, difficult to identify, track and provide DF. The frequency agile radios randomly change their carrier frequency over a very wide bandwidth while they are transmitting at a fixed timing sequence. Thus, most reconnaissance systems in use today, including DF systems, cannot identify, track and provide DF on the LPI RF transmitters because the LPI RF signal disappears before the reconnaissance systems can gather a set of baseline measurements. However, there have been some reconnaissance systems designed to attack the problem of frequency agile LPI RF transmitters. These systems include a full band approach reconnaissance system, a pulse rate detector reconnaissance system and a narrow band fast scan DF reconnaissance system.
The full band approach reconnaissance system is a high end system which utilizes a wideband receiver (or a bank of wideband receivers) that covers the full LPI bandwidth, thus ensuring the intercept of a large percentage of the LPI RF signal pulses (e.g. 50% or more). The full band approach relies on intercepting this large percentage of the LPI RF signal pulses to identify the LPI RF transmitters. Some full band approach reconnaissance systems also include a monopulse DF capability to determine the location of the LPI RF transmitters. The DF or AOA in this type of system is calculated by utilizing a single frequency algorithm. Most of the high end full band approach reconnaissance systems provide tracking of the LPI RF transmitters, LPI RF transmitter type identification, copying of the LPI RF signal, and DF on all LPI RF transmitters. However, the full band approach reconnaissance system has several drawbacks associated with its use which include cost, weight, size and power consumption.
The pulse rate detector reconnaissance system is a low end system which is able to detect the presence of LPI RF transmitters and identify the LPI RF transmitter type. The pulse rate detector reconnaissance system essentially takes a sample of LPI RF signals over the LPI RF transmitter bandwidth and runs a spectral analysis to determine the pulse rate of the LPI RF transmitters. However, one drawback of this system is that if several LPI RF transmitters are transmitting simultaneously at the same pulse rate, this system cannot distinguish between the various LPI RF transmitters. Another drawback the pulse rate detector reconnaissance system suffers from is that it does not contain DF capabilities to determine the location of LPI RF transmitters.
The narrowband fast scan DF reconnaissance system utilizes a narrowband receiver (i.e., 50 KHz) that is stepped in frequency very quickly to capture LPI RF signals. When a LPI RF signal is detected, a fast DF is taken to determine AOA at that particular frequency. This result is then plotted on a screen which displays the frequency versus AOA. As more LPI RF signals are detected at different frequencies and plotted, a line will appear on the screen having the same AOA at various frequencies. This allows the operator to determine the location of the LPI RF transmitter based upon the AOA. However, since the system is not capturing the rising or falling edge of the LPI RF signals, the system cannot provide LPI RF transmitter type identification or separately track multiple LPI RF transmitters.
Each of the current reconnaissance systems are designed to address the problems associated with frequency angle LPI RF transmitters and are capable of providing various information such as tracking, copying, identification and DF information. However, each of these reconnaissance systems have several drawbacks associated with their use that effect their cost, weight, size, power consumption and inability to provide certain functions. What is needed then is a partial intercept LPI reconnaissance system which is capable of providing: identification of LPIRF transmitter types; separate tracking of LPI RF transmitters; and DF on all LPI RF transmitters without suffering from the above-mentioned drawbacks. It is therefore an object of the present invention to provide such a device.